(a) Technical Field of Invention
This invention relates to a toner for developing an electrostatic latent image formed by electrophotography, electrostatic recording process, or the like, to a charge control agent for use in producing the toner, and to a process for preparing the charge control agent.
(b) Background of the Art
As a process for developing an electrostatic latent image formed on an electrostatic image carrier such as an electrophotographic light sensitive member or an electrostatic recording medium, there have been known a wet type developing process of using a liquid developer containing a fine toner dispersed in an electrically insulating liquid and a dry type developing process of using a toner containing a colorant, a magnetic particle, etc. dispersed in a binder resin. The dry type developing process includes a process of using a two-component type developer composed of a carrier and a toner, and a process of using a one-component type developer composed of a toner alone (usually a magnetic toner).
These toners for developing an electrostatic latent image contain a colorant such as a dye or a pigment with binder resin and, with magnetic toners, further magnetic particles etc. Usually, however, these components fail to impart desirable charging properties. Therefore, a charge control agent has been used in the toners. As typical examples of conventionally used charge control agents capable of imparting positive charge to toner particles, there are illustrated electron donative materials such as basic dyes (e.g., Nigrosine dyes, triarylmethane dyes, etc.) and quaternary ammonium salts and, as typical examples of those capable of imparting negative charge to toner particles, there are illustrated metal-containing dyes such as metal complexes of monoazo dyes, chromium-containing organic dyes (e.g., Copper Phthalocyanine Green, chromium-containing monoazo dyes, etc.). However, many of these conventional charge control agents have such problem as that they fail to impart good charge control properties to toner particles over a long period of time due to their insufficient dispersibility in toner particles owing to their poor compatibility or wetting properties with a binder resin, or due to their poor stability or some sublimation properties. Further, some conventional charge control agents are colored and are not suited for color toners.
(c) The Prior Art
As charge control agents capable of solving these problems, there have been proposed metal complexes of salicylic acid and derivatives thereof (e.g., Japanese Examined Patent Publication No. S55-42752, Japanese Unexamined Patent Publication Nos. H7-84412 and H9-34177, etc.) and divalent metal salts of salicylic acid and derivatives thereof (e.g., Japanese Examined Patent Publication No. H7-62766, etc.). Many of these charge control agents are only slightly colored and can be advantageously used in color toners. However, further study is required as to uniform dispersibility in resin and charge control properties.
As has been described above, charge control agents to be used in toner particles for developing an electrostatic latent image are required to have good compatibility or wettability with toner binder resins, good stability, and no colors as well as charge control ability. In addition, they are required to provide toner particles having good storage stability, enough durability to be repeatedly used many times and not to exert adverse influences on fixing property or anti-offset property.
It is, therefore, an object of the invention to provide a toner for developing an electrostatic latent image, which can solve the above-described problems and show excellent properties, a charge control agent having excellent properties, and a process for preparing the charge control agent.
Other objects, features and advantages of the present invention will become apparent from the detailed description of the preferred embodiments of the invention to follow.
As a result of intensive investigations, the inventors have found that trivalent chromium salts of salicylic acid or its derivative produced under specific conditions can satisfy the above-described requirements, thus having completed the present invention based on the finding.
That is, according to the present invention, there is provided a process for preparing a trivalent chromium salt of salicylic acid or its derivative, which comprises adding an aqueous solution of an alkali metal salt of salicylic acid or its derivative of 6.5 to less than 7.0 in pH to an aqueous solution of chromium(III) halide of 3.0 to 5.8 in pH.
Further, according to the present invention, there is provided a charge control agent to be used in a toner for developing an electrostatic latent image, which comprises a trivalent chromium salt of salicylic acid or its derivative prepared by adding an aqueous solution of an alkali metal salt of salicylic acid or its derivative of 6.5 to less than 7.0 in pH to an aqueous solution of chromium(III) halide of 3.0 to 5.8 in pH.
Further, according to the present invention, there is provided a toner for developing an electrostatic latent image, which contains a trivalent chromium salt of salicylic acid or its derivative prepared by adding an aqueous solution of an alkali metal salt of salicylic acid or its derivative of 6.5 to less than 7.0 in pH to an aqueous solution of chromium(III) halide of 3.0 to 5.8 in pH.